1. Field of the Invention
The present invention relates to a pressure-applying fixing roller (hereinafter referred to as a “fixing pressure roller”), and to a fixing device including the fixing pressure roller, which fixing device is suitable for use in an image-forming apparatus.
2. Background Art
A fixing device is employed in an image-forming apparatus such as a copying machine, a facsimile machine, or a laser beam printer. In the fixing mechanism of a fixing device, a heated fixing member (e.g., a heated fixing roller) is rotated in association with rotation of a pressure roller facing the fixing member. In the fixing mechanism, a large nip must be provided between the pressure roller and the fixing member for fixing a toner image on a fed paper sheet by means of pressure and heat. A large fixing width can be achieved by forming the pressure roller from silicone rubber having low hardness. Therefore, the pressure roller is generally formed from silicone rubber foam which has low hardness and low heat capacity (i.e., suppresses loss of heat from a heat source).
There has been proposed a technique for further lowering the hardness of such silicone rubber foam. In the technique, resin microballoons and a volatile component are incorporated into silicone rubber, and, during curing of the silicone rubber, pores are provided between cells through breakage of the resin microballoons or vaporization of the volatile component (see, for example, Patent Document 1). However, this technique (i.e., provision of pores between cells through vaporization of the volatile component) poses problems in that pores are not uniformly distributed, and sufficiently low hardness fails to be achieved.
When a paper sheet is not fed to such a fixing device, operation of the pressure roller is stopped while it is pressed against the fixing member. Therefore, in the case where the pressure roller is formed of a rubber layer of large thickness, a portion of the roller pressed against the fixing member may be deformed, which may cause a problem that defective fixing occurs after resumption of operation of the fixing device. Thus, silicone rubber employed for forming the pressure roller is required not only to have low hardness and low heat capacity, but also to exhibit excellent compression set.
Patent Document 1: Japanese Patent No. 3969942